Electrolytic techniques for recovering silver from waste photographic solutions such as waste fixer have gained favour in recent years. The silver is generally deposited as plate on the cathode, which is periodically removed from the solution for stripping of the silver. Commercial electrolytic recovery equipment abounds but to date recovery of 95% of the available silver has been considered excellent. In practice, the recovery of even 70% is difficult to maintain, let alone exceed.
The principal problem which has arisen in electrolytic recovery of silver from waste photographic solutions has been the precipitation of silver sulphide when the silver concentration, which is typically of the order of 4 gm/l in the untreated solution, falls below 0.5 gm/l. The precipitation of silver sulphide produces a contaminated product and prevents further silver plating, and may be accompanied by the generation of undesirable hydrogen sulphide gas.
It has been recognized that the formation of silver sulphide is associated with an increase in the potential across the electrodes of the electrolyte cell. It has been further appreciated that there is a critical potential across the electrodes at which sulphide formation will commence. These limits are more critical at low silver concentrations, e.g. below 0.5 gm/l. The general approach in the past has therefore been to carefully control the applied current at low silver concentrations. Since the magnitude of the current determines the rate of removal of silver from solution and since setting the current too low will result in commercially unacceptably long recovery times, know electrolytic systems have relied on current settings which were a compromise between speed and efficiency of silver recovery. Moreover, this equipment has entailed careful control of the silver concentration/current density relationship to minimize sulphiding and has therefore required sophisticated sensing devices and feed-back circuits which in many cases have been expensive yet unreliable or ineffective.
Another known approach has been to attempt to maintain an essentially constant flow of solution through the cell while maintaining an essentially constant current density at a level appropriate for the approximate expected average silver concentration. The very nature of these flowthrough cells determines that during operation silver losses occur due to displacement of solution in the cell by input solution from the metering pump. Furthermore, if input silver concentration varies substantially such systems are unable to provide efficient recovery: when the input silver concentration is high, substantial quantities of silver pass through without being recovered, but when the silver concentration is too low, sulphiding occurs.
It is therefore an object of the invention to provide method and apparatus for the electrolytic recovery of metal from solutions which, when applied to the recovery of silver from waste photographic solutions, are able to provide an enhanced silver recovery substantially without sulphiding side reactions.